Japan could miss out on protection under a newly signed American law guarding semiconductor designs against piracy if it doesn't solve a turf battle within its own bureaucracy, the United States has warned.

U.S. Patent and Trademarks Commissioner Gerald J. Mossinghoff said he told officials here on Friday that he would recommend denying coverage for Japanese companies until their government makes firm efforts to guard American designs.

The law, which President Reagan signed on Friday, is designed to protect the American semiconductor industry for the first time from foreign pirates who steal computer chip designs and sell them around the world.

The law also permits companies from countries that have similar legislation or are moving toward it to register their designs for protection in the United States and take action against violators in the U.S. court system.

Commerce Secretary Malcolm Baldridge is charged with deciding which countries will qualify; the patent commissioner is charged with giving a recommendation.

Japanese efforts to comply with the law are caught up in a turf battle between their Ministry of Education, which handles copyright protection, and the Ministry of International Trade and Industry (MITI), which is in charge of patents, according to Mossinghoff and Reagan administration trade specialists in Washington.

"There's not even a decision on which ministry would implement protection," Mossinghoff said.

He added that the intramural dispute could hold up Commerce Department certification that Japan was making a good-faith effort to offer American companies equal protection from computer-chip piracy.

"If the Japanese want to avoid the problem, they have got to get their act together," a Commerce Department expert on Japan added.

American officials had thought that the question of protection from piracy was within MITI's jurisdiction and that Japan was well on its way to enacting legislation that would qualify its chips for protection under the U.S. law.

The differences between MITI and the Ministry of Education surfaced during Mossinghoff's visit to Tokyo last week, a U.S. official said.

U.S. manufacturers, who currently hold the edge in the world's $20-billion-a-year semiconductor industry, had pushed for increased protection for their designs because it had been a legal and common practice for producers to copy one other's designs without permission or payment.

Although Japanese companies have introduced state-of-the-art designs in some fields, they also frequently have copied designs developed at great expense in the United States.

The Semiconductor Chip Protection Act of 1984 gives chip designs 10-year protection similar to those granted by copyrights.

Even before the law's enactment, the Electronic Industries Association of Japan petitioned the U.S. government for protection under it. Japan currently has no such law.

Mossinghoff said he told Japanese officials in Tokyo that "we should see concrete steps toward the enactment of legislation before I would make a [positive] recommendation."